In an aircraft gas turbine (jet) engine, air is drawn into the front of the engine, compressed by a shaft-mounted compressor, and mixed with fuel. The mixture is combusted, and the resulting hot combustion gases are passed through a turbine mounted on the same shaft. The flow of gas turns the turbine by contacting an airfoil portion of the turbine blade, which turns the shaft and provides power to the compressor. The hot exhaust gases flow from the back of the engine, driving it and the aircraft forwardly.
The hotter the turbine gases, the more efficient is the operation of the jet engine. There is thus an incentive to raise the turbine operating temperature. However, the maximum temperature of the turbine gases is normally limited by the materials used to fabricate the turbine vanes and turbine blades of the turbine. In current engines, the turbine vanes and blades are made of nickel-based or cobalt-based superalloys that can operate at temperatures of up to 1900-2100xc2x0F.
Many approaches have been used to increase the operating temperature limits and operating lives of the airfoils of the turbine blades and vanes. The compositions and processing of the materials themselves have been improved. The articles may be formed as oriented single crystals to take advantage of superior properties observed in certain crystallographic directions. Physical cooling techniques are used. In one widely used approach, internal cooling channels are provided within the components, and cooler air is forced through the channels during engine operation.
In another approach, a protective coating formed of an environmental coating or a ceramic/metal thermal barrier coating (TBC) system is applied to the airfoil of the turbine blade or turbine vane component, which acts as a substrate. Such environmental coatings include, for example, simple diffusion aluminides, diffusion aluminides modified by the addition of a noble metal such as platinum, and overlay coatings. The surface of the environmental coating is oxidized to form an aluminum oxide scale that protects the substrate against damage by oxidation and hot corrosion.
This protective layer, with no overlying ceramic layer, is useful in intermediate-temperature applications. For higher temperature applications, a ceramic thermal barrier coating layer may be applied overlying the environmental coating to form a thermal barrier coating system. The ceramic thermal barrier coating layer insulates the component from the combustion gas, permitting the combustion gas to be hotter than would otherwise be possible with the particular material and fabrication process of the substrate.
In the thermal barrier coating application, the ceramic coating must adhere to the substrate during the service life of the protected article, and the individual layers must also remain adhered to each other. If there is a delamination, portions of the coating may spall away from the surface of the protected article. In that event, the article is exposed to the highly aggressive environment of the combustion gases and consequently fails prematurely. There is an ongoing need to reduce the incidence of failures of protective coatings. The present invention fulfills this need, and further provides related advantages.
The present invention provides a modified environmental coating which is more resistant to spallation failures and has improved oxidation/corrosion resistance than comparable conventional coatings. The protective coating of the invention achieves the same protective benefits as conventional coatings, but is less prone to premature failures. The protective coating may be applied by techniques that are known for other applications.
A protected article includes a substrate having a surface, and a protective coating overlying and contacting the surface of the substrate. The protective coating comprises a modified aluminum-containing protective layer having a composition comprising nickel, aluminum, and at least one modifying element selected from the group consisting of calcium, magnesium, and barium, and mixtures thereof, in a total amount of at least about 50 parts per million by weight (ppmw). The modifying element is preferably present in a total amount of from about 50 ppmw to about 300 ppmw, and is most preferably calcium. The modified aluminum-containing protective layer may be, for example, a simple diffusion aluminide, a diffusion aluminide further containing a noble metal, or an overlay coating. Thus, in some cases the modified aluminum-containing protective layer may also comprise elements interdiffused into the modified aluminum-containing protective layer from the substrate.
In a thermal barrier coating system based upon this modified aluminum-containing protective layer, a ceramic layer may overlie and contact the modified aluminum-containing protective layer.
The protective coating is formed by depositing the modified aluminum-containing protective layer overlying the surface of the substrate. The addition of the modifying element to the aluminum-containing protective layer may be accomplished in any operable manner. The modifying element may be deposited before, simultaneously with, or after the deposition of the other elements of the aluminum-containing protective layer. An optional ceramic layer may be deposited overlying the modified aluminum-containing protective layer.
The present approach provides for the inclusion in the modified aluminum-containing protective layer of a modifying element that reacts with free sulfur, such as calcium, magnesium, or barium. Free sulfur, as distinct from reacted and combined sulfur, may diffuse to a surface and, once at the surface, accelerate the spallation of the aluminum oxide scale that is present on the surface of the aluminum-containing protective layer and, if a ceramic layer is present, accelerate its spallation. Loss of the aluminum oxide scale due to spallation results in a decrease in the oxidation resistance of the coated article. Loss of the ceramic layer, if present, results in a loss of its insulating effect. The modifying element reacts with free sulfur to form a sulfide compound, which prevents the sulfur from diffusing to the free surface and consequently inhibits, and desirably prevents, the spallation of the aluminum oxide scale and/or the ceramic layer. The modifying element must be present in an amount sufficient to combine with the free sulfur present near the surface of the substrate and in the deposited layers. It has been found that at least about 50 ppmw of the modifying element must be present to react with the sulfur content that is typically present in such situations.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.